problem is due to code in sun.plugin.URLJarFileCallback, retrieve and download method.
In retrieve, we detected DeployCacheHandler is not avaliable in 1.4.2, and set resourceNotCacheable to false, and contiune to depend on downloadJAR method to download resource to cache. But downloadJAR basically needs DeployCacheHandler in order to download into cache.
So basically when running 6/7 plugin with 1.4.2, plugin works in no-cache mode.
running with 1.4.2 is not of the highest priority right now for JDK 7.
If this needs to fixed in a 6 update release, please open subCR for 6-update.